Element made of composite material including electrical continuity through the element

ABSTRACT

The subject of the invention is an element made of composite material comprising an organic matrix which is reinforced with mineral or organic fibers, as well as at least one assembly with partial superposition of at least two electrically conductive elongate members, which assembly is essentially embedded in the matrix so as to ensure electrical continuity through the element, at least one junction piece, with a conductive coating or made of a conductive metal or alloy, being interposed between superposed portions of said elongate members of each assembly and held in permanent contact with these portions in the composite material by soldering, which links the piece to the elongate members and is preferably carried out during the heat treatment for forming the composite element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation of U.S. patent application Ser. No. 08/570,217filed Dec. 11, 1995, now U.S. Pat. No. 5,863,667.

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject of the invention is an element made of composite material,comprising at least one assembly intended to ensure electricalconductivity through the element, the composite material being of thetype comprising an organic matrix which requires a heat treatment inorder to reach a critical rigidifying and/or shaping temperature and isreinforced with mineral or organic fibres, the critical temperaturebeing a polymerization temperature in the case of a thermosetting matrixand a shaping temperature, for example shaping by moulding, in the caseof a thermoplastic matrix.

The subject of the invention is also the process for manufacturing thiselement, as well as its use in aeronautics

2. Description of the Prior Art

The "metallization" of the elements or pieces made of compositematerials, that is to say the measures taken to ensure electricalcontinuity through these elements or pieces, is a necessary requirementin the aeronautical field in order to ensure, in particular, protectionof the onboard electrical equipments from electromagnetic interferencewhen the aircrafts carrying these equipments are, at least partly,produced using such elements. The object of the invention is, amongothers, to allow the electrostatic charges, which may originate fromelectrical stresses for example, to flow away.

In practice, an electrically conductive mesh is interposed in a stack oflayers of composite materials, these layers being intended to form afuselage panel for example, something which amounts to conferring theproperties of a Faraday cage on the fuselage of an aircraft produced inthis way. Thus, the interior of the fuselage becomes electricallyisolated, the mesh allowing the electric charges to drain away.

However, such a structure turns out not to be sufficient to allowhigh-intensity currents to pass, for example those due to lightning. Inthis case, the electrical discharge may not be completely drained awayby the simple mesh and it runs the risk of adversely affecting thecomposite element in question and of putting the aircraft in danger.

As a general rule, the remedy consists in arranging in addition, in thestack of layers of composite materials, copper shims, generally in theform of narrow tapes.

Experience shows that these shims, inserted into the compositematerials, deform during the heat treatment which the organic matricesof these materials undergo (during the hot polymerization in the case ofthermosetting matrices or during hot shaping and/or moulding in the caseof thermoplastic matrices). During this operation thermal stressesappear. This may result, in particular, in disbondments or fractureswhich adversely affect the characteristics of the elements in question.

This may also result in undulations in the copper strips. Theseundulations have the effect of preventing electrical contact between twosuccessive copper strips. Consequently, such an arrangement may notallow reliable metallization of the elements made of compositematerials.

The current solution necessitates limiting these copper tapes to alength of from 0.8 to 1 m approximately, and ensuring electricalcontinuity by a local overlap by a few centimetres of the copper strips,taken in pairs. As a general rule, this procedure prevents theundulations in the copper tapes during heat treatments.

However, during polymerization or shaping heat treatment, the resins mayinsinuate between the copper bands in the overlap zones and thus impairthe contacts. Consequently, it is no longer possible to ensure properelectrical conduction. It therefore becomes necessary frequently tocarry out work on this kind of assembly in order to effect repairs bysoldering. Provision may also be made-to instal screws or rivets inorder to keep the copper strips in contact in the overlap zones, but theresults are never excellent (oxidation, increased electrical resistance,etc.). In addition, these operations require time and labour and areconsequently expensive.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is to provide an element made of compositematerial, comprising at least one assembly free of all the drawbacks ofthe elements of the prior art; in particular each assembly must allowhigh-intensity electric currents to pass and must not deform or beadversely affected by the action of thermal stresses, especially duringthe operation of polymerizing or hot forming and/or moulding the matrix.

According to the Inventor, surprisingly and unexpectedly, it is possiblenot only to obtain an element made of composite material comprising atleast one assembly ensuring better electrical continuity through theelement than the assemblies of the prior art but, in addition, toachieve this result simply, economically and effectively, from themoment that at least one assembly with partial superposition of at leasttwo electrically conductive elongate members is employed and that ajunction piece, at least coated with a metal or alloy which is a goodelectrical conductor, is interposed between two superposed portions ofthe said elongate members of each assembly, holding the piece inpermanent contact with the said superposed portions.

More precisely, the element made of composite material in accordancewith the invention, comprising an organic matrix which has a criticalrigidifying and/or shaping temperature and is reinforced with mineral ororganic fibres chosen, in particular, from the group comprising carbon,glass and aramid fibres, as well as at least one assembly with partialsuperposition of at least two elongate members of a first electricallyconductive metal or alloy having a melting point substantially greaterthan the said critical temperature of the matrix, the assembly beingessentially embedded in the matrix so as to ensure electrical continuitythrough the element, is characterized by the fact that at least onejunction piece, consisting at least partly, on the surface, of a secondelectrically conductive metal or alloy having a melting pointsubstantially less than the melting point of the said first metal oralloy, is interposed between superimposed portions of the said elongatemembers of each assembly, with which portions the said junction piece isheld in permanent contact by soldering the said second metal or alloy tothe said first metal or alloy.

According to a preferred embodiment, the elongate members of the firstmetal or alloy are foils, tapes or strips, known per se.

The reason for this is that such elongate members allow the electriccharges to drain away properly and, at the same time, can be easilyintegrated into the composite materials.

According to one advantageous embodiment, the junction piece comprises asupporting structure, preferably in the form of a substantially planepiece, made of a third electrically conductive metal or alloy, having amelting point substantially greater than the critical temperature of thematrix, and including an at least partial coating made of the saidsecond metal or ally whose melting point is substantially less than themelting point of the said third metal or alloy. This makes it easierboth to integrate the junction piece into the composites and to obtaingood bonding, by soldering, of the coating made of the said second metalor alloy of the junction piece to the said first metal or alloy of theelongate members.

Furthermore, in this way, it may be even more certain that the electriccharges drain away, rapidly and effectively. It is preferred to employ ajunction piece whose plane piece of the third metal or alloy takes theform of a grid, a mesh or a trellis, at least partly coated with thesecond metal or alloy, preferably having a thickness of between 0.05 mmand 2 mm. Plane junction pieces are thus obtained which are particularlysuitable for ensuring good electrical conductivity and properdraining-away of the charges, in particular if the third metal or alloyis chosen from the group comprising the alloys of copper, of aluminiumand of nickel, whereas the first metal or alloy is chosen from the groupcomprising copper, aluminium, stainless steel and nickel, and the secondmetal or alloy is chosen from the group comprising indium, nickel, tin,lead, zinc and eutectic alloys, especially of the aforementioned metals.In addition, such a junction piece is simple to arrange in the stack ofthe composite layers of the element to be produced.

It is particularly advantageous to produce an element having coppershims as the elongate members and a mesh made of bronze impregnated withindium as the junction piece between two superimposed portions of thecopper shims. The reason for this is that these metals are particularlygood conductors and are inexpensive, and indium, which has a low meltingpoint (156.2° C.), is simple to deposit, as a coating on the bronze forexample, and use in order to produce a low-melting-point solder joint.

In an embodiment providing a better quality of soldered joint connectingthe junction piece to the elongate members, and which is economical interms of manufacturing time, as it avoids having to deposit a coatingmade of the said second metal or alloy on a supporting structure, suchas a mesh, made of the said third metal or alloy, the said junctionpiece consists just of the said second metal or alloy. Advantageously,the said junction piece is a flat and thin wafer of this second metal oralloy, so as to present homogeneous surfaces for bonding to the elongatemembers, and this wafer is arranged in a window made in a layer ofreinforcing fibres, which layer is also interposed between thesuperimposed portions of the elongate members to which the wafer issoldered.

Each assembly of at least two elongate members and of at least onejunction piece may be produced in a first step and then inserted, in asecond step, into a stack of composite layers before the heat treatmentfor rigidifying and/or shaping the stack. However, it is particularlyadvantageous, in order to achieve a considerable reduction in theduration and complexity of manufacture, and therefore also in the costs,for the melting point of the second metal or alloy to be substantiallyless than the critical temperature of the matrix. This makes it possibleto insert the elongate members and the junction pieces, which are notsoldered together, into the stack of the composite layers of the elementbefore the heat treatment for rigidifying and/or shaping the stack, andto solder the second metal or alloy of the junction pieces to the firstmetal or alloy of the elongate members during the heat treatmentallowing the critical temperature of the matrix to be reached, thisbeing necessary for it to become rigid and/or to be shaped, in order toobtain a finished and usable composite element.

The element made of composite material according to the invention may beused in particular in the field of aeronautics and especially for themanufacture of helicopter rotor blades or hubs or for the manufacture offuselage or cowling panels, hatches or doors of aeroplanes and ofhelicopters.

The process for manufacturing an element according to the invention ischaracterized by the fact that:

in a first step, at least one junction piece is produced by making it atleast partly, on the surface, from the second electrically conductivemetal or alloy, this being chosen in particular from the groupcomprising indium, nickel, tin, lead, zinc and eutectic alloys, inparticular of the aforementioned metals;

in a second step, at least one assembly is prepared by interposing atleast one junction piece obtained according to the first step betweensuperimposed portions of at least two elongate members produced in thefirst electrically conductive metal or alloy, this being chosen inparticular from the group comprising copper, aluminium, stainless steeland nickel; and

in a third step, the element is produced by inserting at least oneassembly prepared according to the second step into a composite materialand by carrying out a heat treatment enabling the critical temperatureof the matrix of this material to be reached. This heat treatment isthat required by the matrix, that is to say a polymerization or shapingheat treatment depending on whether the resin or resins of the matrix isor are thermosetting or thermoplastic, and, possibly and simultaneously,a heat treatment for joining the junction piece or pieces to theconductive elongate members by soldering the second metal or alloy tothe first, if the second metal or alloy has a melting pointsubstantially less than the said critical temperature of the matrix.

According to a first variant of the process of the invention, in thefirst step, the junction piece is produced by depositing, for exampleusing a wave technique, by means of a soldering iron, a coating of thesecond metal or alloy on a supporting structure shaped as asubstantially plane piece produced in the said third electricallyconductive metal or alloy and chosen in particular from the groupcomprising the alloys of copper, of aluminium and of nickel.

According to a second variant of the process, in the first step, thejunction piece is produced by compressing and/or rolling at least onelittle piece of the said second metal or alloy into a substantially flatand thin wafer and, in the second step, before superposition, one withrespect to the other, of the said superimposed portions of the twoelongate members, the wafer is arranged in a window made in a layer ofreinforcing fibres, which layer is arranged on one of the saidsuperimposed portions.

According to an even more preferred embodiment, in the second step ofthe process, at least one junction piece is held between superimposedportions of the elongate members by means of an adhesive or adhesivelybonding substance which, in the second variant of the process, may be aresin for impregnating a small strip of a layer of reinforcing fibrescovering the window receiving the wafer, this resin being of the sameclass as the resin or resins of the matrix of the composite material.

In this way, the manufacture of the element made of composite materialis easier since correct positioning of the junction piece between thesuperimposed portions of the elongate members is ensured.

According to an even more advantageous embodiment, in particular for theformation of the composite material, in the third step of the process,the heat treatment is carried out at an underpressure of betweenapproximately 0.05 MPa and approximately 0.01 MPa in a vacuum bag withinan autoclave, firstly at a temperature of between approximately 60° C.and approximately 120° C., for a duration of between approximately 0.5 hand approximately 1.5 h, and then at a temperature of betweenapproximately 140° C. and approximately 220° C., for a duration ofbetween approximately 1 h and approximately 3 h and at a pressure ofbetween approximately 0.15 MPa and 0.55 MPa.

This heat treatment is particularly economical and effective. It alsoturns out that the apparatus used for implementing it is a standardapparatus, which therefore does not require any special installations.

It should also be noted that the required conditions, especially thoseof pressure and temperature, are conditions which are relatively easy toachieve.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will be even more clearly understood with the aid of thenon-limiting examples which follow and in which two advantageousembodiments of the invention are indicated. These examples are describedwith reference to the appended drawings in which:

FIG. 1 is a diagrammatic perspective view representing the preparationof a first example of a junction piece;

FIG. 2 is a view similar to FIG. 1 representing the preparation of anassembly with the aid of the junction piece of FIG. 1;

FIG. 3 is a diagrammatic cross-sectional view of a laminated elementmade of composite material incorporating an assembly according to FIG.2;

FIG. 4 shows, at a) and b), temperature and pressure curves as afunction of time, corresponding to the heat treatment for forming theelement made of composite material of FIG. 3;

FIG. 5 is a curve of the measurement of the electrical resistance as afunction of the distance in the element of FIG. 3;

FIGS. 6a to 6c are three plan views representing three successive phasesin the preparation of an assembly with the aid of a second example of ajunction piece;

FIG. 7 is a diagrammatic longitudinal sectional view of the assembly ofFIG. 6c.

DETAILED DESCRIPTION OF THE INVENTION

The example described with reference to FIGS. 1 to 3 relates to themanufacture of a laminated element made of composite material,comprising copper shims or tapes joined together by a bronze meshcovered with indium, in order to ensure electrical continuity in theelement.

It is proposed to manufacture an element made of composite material inaccordance with the invention which comprises a synthetic organic matrixmade of a thermosetting resin, of the so-called "180° C. class", that isto say the polymerization temperature of which is 180° C., such as anepoxy resin, reinforced with glass fibres and carbon fibres insuperimposed layers, as well as an assembly with partial superpositionof two copper shims or tapes approximately 40 mm wide, between which abronze mesh partially coated with indium is interposed.

In a first step, the impregnation or coating of a bronze mesh is carriedout using indium. To do this, a substantially square bronze mesh 1, eachside being approximately 30 mm, is fluxed using a product for fluxingcopper or copper alloys before soldering, such as the one marketed underthe name HARAFLUX, and then the bronze mesh is thoroughly rinsed in hotwater and dried in air at 60° C.

The mesh 1 thus prepared is placed on a plane wooden support 2, as shownin FIG. 1. The mesh 1 is held on the support 2 by means of asingle-sided adhesive 3, for example made of PTFE, adhesively bonded tothe support 2 and upon which the mesh 1 is held in place by means of asingle-sided adhesive 4 adhesively bonded along two opposite sides ofthe mesh 1 and on those parts of the support 2 which are adjacent tothese two opposite sides.

The mesh 1 coated with indium 6 is obtained by deposition using a wave,passing an electric soldering iron having a wide bit 5 over the mesh 1,pushing in front of it an indium wave 6a over the mesh 1. Since themelting point of indium, a good electrical conductor, is less than thatof bronze, the bronze mesh 1 is impregnated or coated with a thin layerof indium 6, the thickness of which may be between 0.05 mm and 2 mm.

In a second step, an assembly 10 is produced by interposing the mesh 1impregnated with indium 6 between two copper tapes or shims 7, as shownin FIG. 2.

To do this, two 40 mm-wide copper tapes or shims 7 are cut to thedesired length and straightened out in order to remove possibleundulations.

A portion 8 on one of the surfaces of each copper shim 7 is brushed inits end part which will come into superposition with the end part of theother one. Adhesively bonded to the brushed portion 8 of one of the twocopper shims 7 is a double-sided adhesive film 9, for example made ofsilicone, having a square window in its central part, then the foursides of the mesh 1 impregnated with indium 6 are adhesively bonded to apart of this adhesive 9 around its window and then the brushed portion 8of the other copper shim 7 is adhesively bonded to that part of theadhesive 9 which is not covered by the mesh 1, as indicated in FIG. 2.

The two superimposed ends 8 of the shims 7 are joined together by theadhesive 9 with interposition of the mesh 1 coated with indium 6. Theassembly 10 may be subjected to a heat treatment, at an underpressure of0.03 MPa below atmospheric pressure (absolute pressure of 0.07 MPa) in avacuum bag, within an autoclave, firstly at a temperature of 80° C. for1 hour-and then at 180° C. for 2 hours at an overpressure of 0.2 MPa, asshown in curves a) and b) in FIG. 4. However, the heat treatment ispreferably carried out after putting the assembly 10 in place in a stackof layers 11 and 12 of reinforcing fibres preimpregnated with theaforementioned synthetic resin, which can polymerize at 180° C., inorder to form an element 13 made of composite material by the hotpolymerization of the resin and by compacting the stack under pressure,as described hereinbelow in the third step, with reference to FIG. 3.

In this third step, the assembly 10 thus obtained is inserted between,on the one hand, three upper layers 12 of carbon fibres and, on theother hand, a lower layer 11 of glass fibres which are preimpregnatedwith epoxy resin, the fibres being made up in the form of cloths,braids, laps or rovings, these being unidirectional or crossed,depending on the structure sought for the element 13 made of compositematerial which it is desired to obtain (see FIG. 3). The element 13 thusobtained is subjected to a heat treatment carried out in a vacuum bag atan underpressure of 0.03 MPa (absolute pressure of 0.07 MPa) and withinan autoclave, firstly at a temperature of 80° C. for 1 hour and then ata temperature of 180° C. for 2 hours, at an overpressure of 0.2 MPaafter the end of the 80° C. temperature hold.

The heat treatment is shown diagrammatically by curves a) and b) in FIG.4. In FIG. 4, a), the ordinate axis represents the temperature T (in °C.) and the abscissa axis represents the treatment time t (in hours). InFIG. 4, b), the ordinate axis represents the pressure P (in MPa) and theabscissa axis represents the treatment time (in hours).

According to FIG. 4, b), the underpressure of 0.03 MPa in the vacuum bagcontaining the laminated element 13 is maintained continuouslythroughout the duration of the heat treatment. At the beginning of thepolymerization cycle, according to FIG. 4, a), the temperature rise,above room temperature, is substantially linear up to a temperature of80° C. and is then held constant for 1 hour. A little before the end ofthis 80° C. temperature hold, the pressure is increased substantiallylinearly in the autoclave, around the vacuum bag, above atmosphericpressure until an overpressure of 0.2 MPa is reached, which is then heldconstant (see FIG. 4, b)). At the beginning of this overpressure hold,according to FIG. 4, a), the temperature is increased linearly from 80°C. to 180° C. in order to polymerize the resin, and is then heldconstant at this temperature for approximately 2 hours. Next, thistemperature decreases substantially linearly from 180° C. to roomtemperature, and the overpressure is reduced by 0.2 MPa to atmosphericpressure during this cooling phase. Since the melting point of indium(156.2° C.) is less than the 180° C. polymerization temperature, theindium coating 6 of the bronze mesh 1 melts on coming into contact withthe copper of the tapes 7 in the assembly 10 and then solidifies oncooling, ensuring a soldered joint fastening the two tapes 7 to themesh 1. Finally, the underpressure in the vacuum bag is brought to zero,and the autoclave and then the vacuum bag may be opened in order toremove the laminated composite element thus manufactured. It should benoted that since the melting points of copper and bronze are very muchgreater than the polymerization temperature of the resin, the heattreatment does not damage the tapes 7 or the mesh 1.

The good electrical conductivity through the element 13 is illustratedby the graph in FIG. 5.

The graph represents the variation in electrical resistance as afunction of the distance between a reference point and a measurementpoint moved along the copper tapes 7 on the element 13. It may be seenthat the measured electrical resistance is virtually linear, includingin the region of the junction zone 14 where the junction piece formed bythe mesh 1 coated with indium 6 is located.

The result of this is that the assembly 10 as designed within the scopeof the present invention allows good electrical continuity within anelement 13 made of composite material, including in the region of thejunction piece (1, 6), by virtue of the interaction of the latter (madeof two materials which are good electrical conductors--bronze andindium) with the copper strips or tapes, which are good electricalconductors.

As a variant, the junction piece may be a bronze mesh coated with aknitted nickel fabric, nickel also being a good electrical conductor.

The construction of a second example of joining copper tapes together byusing a junction piece, in order to ensure electrical continuity in anelement made of composite material having an organic matrix, isdescribed hereinbelow with reference to FIGS. 6a to 7. It is assumed,hereinafter, that a composite element such as 13 in FIG. 3 ismanufactured in a mould of conventional structure, in two parts havingcomplementary internal cavities. The process starts by laying down, asshown diagrammatically in FIG. 6a, in the internal cavity of the lowerpart (not shown) of the mould, the layer 11 in FIG. 3, for example alayer of glass-fibre cloth, this layer being intended to form theexternal skin of the element 13, having a thickness of 0.05 mm and beingimpregnated with the epoxy resin polymerizing at 180° C. Laid onto thislayer 11 is a first copper strip 7 after it has been straightened out,as in the previous example, in order to remove undulations, abraded inits end part 8 by mechanical brushing, for example by using a metalsponge, and then degreased, the copper strips 7 being handled withgloves in order to prevent any oxidation of the copper. Next, a littlesquare or rectangular piece 14 is cut out in a layer of reinforcingfibres, which may be the same resin-impregnated glass-fibre cloth as thelayer 11, and laid over the end 8 of the copper strip 7 and over theadjacent parts of the glass cloth 11, after having cut out asquare-shaped window 15 in this little piece 14, which is adhesivelybonded in places to the layer of cloth 11, around the end 8 of the strip7 which this little piece 14 covers. This adhesive bonding may beprovided by the impregnation resin of the little piece 14 of glasscloth. Compared to the width of the copper strip 7, of the order of from30 to 40 mm, the dimensions of the little piece 14 of glass cloth aresuch that those parts of this little piece 14 which are applied againstthe glass cloth 11 have a width of from 10 to 15 mm, on the three sidesaround the end 8 of the strip 7, and such that the window 15 issurrounded, on these three sides, by a part of the little piece of glasscloth 14 covering the copper strip 7 over a width of from approximately5 to 7 mm.

As shown in FIG. 6b, a square-shaped indium wafer 16 is then laid on theend 8 of the copper strip 7 and in the window 15 of the little piece ofglass cloth 14. This wafer 16 is smaller than the window 15 cut out inthe little piece of glass cloth 14 so as to leave a gap of fromapproximately 5 to 7 mm between the sides of the wafer 16 and those ofthe window 15.

The indium wafer 16 has been prepared by compressing a little piece ofpure indium in a press or by rolling it between rolls, in order toconvert this little piece of indium into a wafer, after having protectedit by placing it between two films of a synthetic material of the typeused for making vacuum bags, so as to prevent the indium from adheringto the jaws of the press or to the rolling rolls. The indium wafer thusformed has a thickness of between approximately 0.15 mm and 0.25 mm. Cutout from the flat and thin wafer thus obtained is the square-shapedwafer 16, having dimensions slightly smaller than those of the window 15which receives it, as explained hereinabove.

If appropriate, in order to keep the wafer 16 in place in this window15, a small strip 17, for example from 5 to 7 mm in width, ofresin-preimpregnated glass cloth, this small strip therefore beingadhesive, is laid on top of the wafer 16 and the little piece of glasscloth 14, transversely to the copper strip 7, as shown in FIG. 6b.

Next, the second copper strip 7 is laid down, as shown in FIG. 6c, alongthe extension of the first strip 7, so that its end 8, having a surfaceprepared by being abraded and degreased, is placed on top of the stackconsisting of the small strip of cloth 17, the indium wafer 16, thesmall piece of glass cloth 14 and the facing end 8 of the first copperstrip 7. The two copper strips 7 are thus superimposed by their ends 8,as in the first example (see FIG. 2), and then the whole assembly iscovered with a second little rectangular or square piece 18 of the sameresin-impregnated glass cloth in order to stabilize the assembly 10'thus prepared and, in particular, to keep the copper strips 7 insuperposition by their ends 8. The assembly thus prepared is shown insectional view in FIG. 7. It is compacted by being placed in a vacuumbag connected for a minimum of one hour to a vacuum source in order tohold together all the components thus positioned. After the compactingoperations the laying-up is carried out according to the structure whichit is desired to obtain, for example by stacking, on top of the copperstrips 7 and the assembly 10' of FIG. 7, three layers of carbon-fibrecloth 12, as shown in FIG. 3, if it is desired to produce an elementmade of composite material having the same strong structure as that inFIG. 3.

The stack thus obtained is then subjected to the heat treatment ensuringpolymerization of the 180° C. resin. This heat treatment may be thatdescribed herein-above with reference to FIG. 4. During this heattreatment, the indium of the wafer 16 melts at the temperature of 156.2°C. and fills the window 15, before polymerization at 180° C.

After cooling, as in the first example, the indium wafer 16, forming ajunction piece between the two copper strips 7, is soldered to thecopper of the strips 7 via homogeneous surfaces, providing a solderedjoint of good quality.

As mentioned hereinabove, instead of indium, it is possible to use forexample nickel, tin, lead, zinc or eutectic alloys of these metals,whereas the copper strips may be replaced with for example aluminium,stainless steel or nickel strips or tapes.

When the junction piece is not entirely produced in thelow-melting-point metal or alloy used to solder together the conductivestrips or tapes, it is possible to use, in order to produce the planestructure carrying the low-melting-point coating, and instead of bronze,aluminium and/or nickel alloys as well as other copper alloys forexample.

The invention may be applied to the manufacture of any element made ofcomposite material which must exhibit electrical continuity, and inparticular those which can be used in aeronautical construction toproduce fuselage, wing or cowling panels, fuselage or wing doors andhatches, control-surface elements or box-section elements for aeroplanesand helicopters, as well as hubs or blades of helicopter rotors forexample, without this list being limiting.

What is claimed is:
 1. Process for manufacturing an element, saidelement comprising an organic matrix having a critical temperaturereinforced with fibers; and at least one assembly comprising a partialsuperposition of at least two elongate members of a first electricallyconductive material having a melting point substantially greater thansaid critical temperature of the organic matrix, said at least oneassembly being essentially embedded in the organic matrix; at least onejunction piece having a surface comprising a second electricallyconductive material having a melting point substantially less than themelting point of said first metal or alloy, said at least one junctionpiece being interposed between superimposed portions of said at leasttwo elongate members of said at least one assembly, said superposedportions holding said at least one junction piece in substantiallypermanent contact by soldering said second material to said firstmaterial, wherein:in a first step, at least one junction piece isproduced by making it at least partly, on the surface, from said secondelectrically conductive metal or alloy; in a second step, at least oneassembly is prepared by interposing at least one junction piece obtainedaccording to the first step between superimposed portions of at leasttwo elongate members produced in the first electrically conductive metalor alloy; and in a third step, the element is produced by inserting atleast one assembly prepared according to the second step into acomposite material and by carrying out a heat treatment enabling thecritical temperature of the matrix of the said material to be reached.2. Process according to claim 1, wherein, in the first step, thejunction piece is produced by depositing a coating of said second metalor alloy on a substantially plane piece produced in the thirdelectrically conductive metal or alloy.
 3. Process according to claim 2,wherein, in the first step, the coating is deposited, using a wavetechnique, by means of a soldering iron for example.
 4. Processaccording to claim 1, wherein, in the first step, the junction piece isproduced by compressing and/or rolling at least one piece of said secondmetal or alloy into the form of a substantially flat and thin wafer ofsaid second metal or alloy.
 5. Process according to claim 4, wherein, inthe second step, the wafer is arranged in a window made in a layer ofreinforcing fibres, which layer is arranged on one of the superimposedportions of the two elongate members before superposition by the otherof the said superimposed portions.
 6. Process according to claim 1,wherein, in the second step, at least one junction piece is held betweensuperimposed portions of the elongate members by means of an adhesive oradhesively bonding substance.
 7. Process according to claim 1, wherein,in the third step, the heat treatment is carried out at an underpressureof between approximately 0.05 MPa and approximately 0.01 MPa in a vacuumbag within an autoclave, firstly at a temperature of betweenapproximately 60° C. and approximately 120° C., for a duration ofbetween approximately 0.5 h and approximately 1.5 h, and then at atemperature of between approximately 140° C. and approximately 220° C.,for a duration of between approximately 1 h and approximately 3 h and ata pressure of between approximately 0.15 MPa and 0.55 MPa.